Mobility of underwater vehicles is governed by various factors. A vehicle's density and gravity play a great role in underwater mobility. Underwater vehicles must have the means to counter the effect of their gravity and/or buoyancy in order to swim smoothly through a water column and perform tasks efficiently. An underwater vehicle having a neutrally buoyant state is the optimum condition for swimming and transporting through the water column. In some applications, such as visual inspection of underwater structure or video shooting of underwater target, free swimming with neutral buoyancy is desired. On the other hand, an underwater robot having a negatively buoyant state is desired for crawling or anchoring on the seabed. Some applications, such as fouling cleaning, robotic arm manipulation and maintenance require the underwater vehicle to be stable and heavy on the subsurface floor to overcome the counter effect of operations (i.e., equal and opposite reactionary forces).
The concept of buoyancy control was developed with the earliest evolvement of submarines. Submarines typically change their internal buoyancy by varying their volume underwater using pumps and gas cylinders, i.e., hydraulically. However, hydraulic buoyancy control systems are usually bulky, complicated and optimized for large size, deep underwater vehicles.
Accordingly, there is a need to provide underwater vehicles with a means of controlling buoyancy with an easy to manufacture, operate, and maintain, cost-effective and compact system that can be applied for specific applications and conditions. By utilizing the water surface buoyancy limit and the seabed gravity limit, discrete buoyancy control can be achieved according to the present invention as disclosed herein.